1. Field of the Invention
This invention relates to the control of processes and processing equipment, and in particular production processes/equipment for use in the pulp and paper industry. The invention is particularly advantageous for pulp digesters.
2. Background of the Invention
Processors storing large quantities of chemically reactive (and some cases reacting) fluids are utilized extensively throughout the paper and pulp industry. Typically, the processes and equipment are “in-line” with the output from one processor providing the input to a subsequent downstream processor. Each processor in the paper and pulp industry processes a large tonnage of product. For example, a continuous digester, can process more than 1,300 tons per day of digested pulp. Output from a pulp digester becomes input to downstream processors such as for example oxygen delignification processors, bleaching processors, and causticizers.
FIG. 1 is a schematic diagram depicting a number of in-line processing units processing raw pulp products toward a final output product suitable for paper production. FIG. 1 shows specifically a chemical pulp manufacturing process flowing from left to right with output from a digester 2 flowing to an oxygen delignification processor 4 and then to a bleaching processor 6. The spent liquid effluent extracted from the digester 2 is feed “off-line” to a causticizer 8 which restores the spent liquid effluent to a proper alkali concentration before return to the digester 2. The causticizer 8 itself represents another processor (e.g. a vat processor containing large quantities of chemically reactive fluids whose output product must be controlled to a desired standard in the manufacturing line). With this type of arrangement or system, deviations from target specifications of the resultant product from any one stage can impact the downstream processors.
Continuous pulp digesters are very complex vertical reactors (typically tubular) used in the pulp and paper industry to remove lignin from wood chips. Usually, continuous digesters are separated into multiple reaction and extraction zones. Optimal control of a digester can be difficult due to long dead times in which changes to input process variables are not immediately apparent. When a process parameter is changed or a step commences, e.g., by the addition of a material such as an alkali or by affecting a temperature change, the end effect is not immediately apparent due, e.g., the time required to realize the effect and the inertia of the system. The time from when a change occurs to the point at which the effect is realized, fully or partially, can be referred to as “dead time.”
In order to yield large or more optimum production quantities of digested product and to be economical with a minimum of chemicals and energy usage, the process must be controlled to maintain optimum cooking conditions throughout the digester to ensure selective delignification while simultaneously optimizing pulp quality and production costs. To facilitate control, reliable pulp quality measurements are often used to provide accurate real-time information. Indeed, certain basic control and quality measurements—Kappa, pulp strength, and chemical residuals—have been made regularly for decades. In the past, analyses of these properties were made off-line in the laboratory, but such analyses were slow and error-prone. However, with recent advances in measuring technologies, these analyses have been extensively automated such that measurements can be made on-line. To maximize the impact of automated measurements, there is a need for efficient controls and/or control methods that are easy to modify, tune, and configure, and yet can handle the complexity of continuous digester processes.
As a consequence of the heterogeneities in the feedstock, i.e. the wood pulp, a digester undergoes constant changes due to the complicated structure and properties of the various wood pulps being fed to the digester. Besides differences in the pulp feedstock from one particular batch of wood chips to another, even the moisture content of the chips being fed into the digester can vary by as much 30% during a single day's production. Further, the large amounts of wood pulp and chemicals contained in the digester create a “chemical inertia” which makes instantaneous changes to the digesting conditions, such as for example changes in alkali concentration, cooking temperature, and white liquor concentration, difficult if not impossible to rapidly adjust. As a consequence, it is generally impossible to describe the dynamics of digester with precise mathematical models. Furthermore, a typical retention time for the pulp in a digester can in some cases exceed five hours. Due to possible channeling (i.e., unexpected changes in plug flow in the tubular reactor) or other unexpected disturbances, it is impossible to estimate the retention time accurately for a particular pulp product flowing through the digester.
As noted, a digester can process more than 1,300 tons per day of digested pulp. Maximizing pulp production at a specified Kappa number using a minimum input of chemicals and energy and a minimal waste discharge is highly desirable in order to produce an efficient pulp digesting process. In a digester, lignin is removed from for example wood chips. Lignin is the naturally occurring bonder in a wood product which bonds the wood fibers together. An aqueous solution of the sodium hydroxide and hydrosulfide (i.e., white liquor) is used to react (i.e., to digest) the wood products inside the digesters thereby dissolving the lignin from the wood product.
Presently, a titration method is a known and commonly used to measure a Kappa number of various pulps. This titration method is described in Tappi Test Methods—T236 cm-85, Tappi Press, 1996, the entire contents of which are incorporated herein by reference. Using the titration method, a pulp Kappa number is calculated using the difference between the initial volume of potassium permanganate blank solution and the final volume of potassium permanganate remaining after oxidation of lignin in the pulp-permanganate solution. For example, the digestion of wood chips in an alkali solution and the resulting pulp Kappa number obtained using a permanganate solution are both described in Bentvelzen et al. (U.S. Pat. No. 4,216,054), the entire contents of which are incorporated herein by reference. Kappa number is not the only one way to measure lignin, e.g. others like K-number, P-number and others known in the art can be used.
Prior to entry into the digester, wood chips are typically cooked and steamed (to remove air from the pores of the chips) and fed into an impregnation vessel together with the white liquor. While in the impregnation vessel, white liquor penetrates the chips, and the chips are subsequently carried into a top section of the digester where a mixture of the wood chips and the white liquor is brought to a desired reaction temperature. In a top section of the digester, the chips react with the white liquor to digest the lignin, and spent liquor (i.e., that liquor which has been depleted of its alkalinity by the chemical reaction with the lignin) is extracted as the digested chips migrate into lower cooking sections. Fresh white liquor is added to further continue the delignification process. The blow Kappa number of the digested (i.e., reacted) product can be assessed from a blow-line (i.e., an exit line) in which the Kappa number provides a measure of how effectively the lignin has been digested from the wood fiber.
As disclosed for example in Beller et al. (U.S. Pat. No. 5,032,977), the entire contents of which are incorporated herein by reference, to address the complexities of controlling a wood digester, “model” based control processes have been developed. In a model-based control process, a model assumes the input properties of the pulp product entering the digester, calculates expected values for the resultant properties of the digested product, and alters the process variables of the reactor (e.g., the pulp product feed rate, the alkali input feed, and the digester temperature) to affect the resultant properties. A model based approach is a complex approach requiring complicated calculations if any kind of reliable prediction of the reactor is to be made. Yet, for the above-noted reasons, pulp digesters are not simple chemical fluid beds conducive to model based predictions. Initial assumptions of input properties and the resulting models of the digester are susceptible to variations of the input properties and are susceptible to unexpected changes in the product flow through the large digester (i.e., the above-noted channeling). When unexpected changes occur, model based controls have no way to recognize that the unexpected changes may be spurious. The model based controls consequently improperly compensate the input process variables, thus producing control oscillations and instabilities in the output properties of the digested wood product.
While model based controls, such as those described by Beller et al. for example, can use adaptive control to learn and refine the process control model, the learning process needs to be based on at least a quasi-steady state condition maintained in the reactor. Otherwise, what is learned is in error. Indeed, in those models which use adaptive control, a disturbance to the steady state operation can result in the models being temporarily skewed, as the “learned” refinements are not representative of the process when unexpected disturbances occur. As a result, when unexpected disturbances occur, once again a series of oscillations in the model-based control occurs, producing process control instability.
The problems illustrated above for a pulp digester extend to other paper mill processes listed above such as for example the oxygen delignification processors, the bleaching processors, and the causticizers, and in general are prevalent in any chemical processor in which imhomogeneities in input feedstock, the chemical inertia of the process reactor, and/or the fluid flow make problematic the accurate prediction of future changes following changes to input parameters.